1. Field of the Invention
The present invention relates to an optical imaging system, a camera device, and a hand-held data terminal device including the optical imaging system. The optical imaging system is applicable to a silver film camera, a digital camera, and a video camera. It is also applicable to a hand-held data terminal device.
2. Description of the Related Art
Along with widespread of a digital camera, users have come to have diverse demands for the digital camera. Particularly, users have desired development of a high image quality, compact-size camera incorporating a high-performance, large-aperture (small f-number) single focus lens.
Such a compact-type, single focus lens camera is required to exert a high optical performance with a resolution equivalent to an image sensor with at least 10 to 20 million pixels. Furthermore, it is required to reduce coma flares with an aperture fully opened, prevent blurs in point images in the periphery of angle of view at a high contrast, prevent unwanted colors in an image portion with a large difference in brightness with less chromatic aberration, and generate images with less distortion.
In terms of the large aperture, in order to distinguish such a single focus lens camera from a general compact-size camera incorporating a zoom lens, it is required to have an f-number of at least 2.4 or less, or preferably 2.0 or less. Most users prefer a photographic lens of a wide angle of view so that a half angle of view of a focus lens should be preferably 38 degrees or more. The half angle of view of 38 degrees is equivalent to a focal length of 28 mm of a 35 mm silver film camera.
Due to property of an image sensor having a color filter and a micro lens arranged on each pixel, a digital camera is preferably configured that an exit pupil position is distanced away from an image plane to make ambient light incident on an image sensor at a substantially vertical angle. As a typical example of a wide-angle, single focus lens, a so-called retrofocus type lens system is often adopted in such a digital camera. In the retrofocus type, a lens group having a negative refraction and a lens group having a positive refraction are arranged on an object side and on an image side, respectively.
The retrofocus type lens system generally performs rear focusing. However, it has a problem of tilting an image plane when focusing. With a larger aperture, especially, the tilt of an image plane largely affects the quality of images. In view of solving such a problem, a focusing method by moving a plurality of lens groups has been proposed. Japanese Patent No. 3629191 (Reference 1) and Japanese Laid-open Patent Publication No. 6-324264 (Reference 2) disclose an optical imaging system using such a focusing method to achieve a large aperture and a wide angle of view to some extent, for example. The optical imaging system in Reference 1 realizes a half angle of view of 27 degrees and f-number being 2.8 or more but the light amount is relatively small.
The optical imaging system in References 1 and 2 are configured to focus on an object in a close range from infinity by moving lens groups before and after the aperture diaphragm together to an object side so that intervals between the lens groups are reduced. In other words, a front lens group is protruded to the object side at focusing, increasing the total length of the lens system. This leads to preventing downsizing of an imaging device.